1. Field of the Invention
The present invention relates to liquid crystal devices for use in liquid crystal displays, liquid crystal light shutter, etc. and, in particular, to a liquid crystal device the gradation drive characteristics of which been improved by overcoming the problem of hysteresis in electro-optical response of the liquid crystal, and to a method of manufacturing such a liquid crystal device.
2. Description of the Related Art
In a typical liquid-crystal-device cell construction, the liquid crystal is held between two opposed electrodes, to which a voltage is applied so as to change the orientation of the liquid crystal molecules, thereby controlling the light transmittance of the cell.
In a cell using a TN (twisted nematic) liquid crystal, the transmittance varies in an analog fashion according to the value of the applied voltage, so that a gradated (gray scale) display is possible.
A ferroelectric liquid crystal is a liquid crystal material having an excellent responsiveness and providing a wide viewing angle. A display device utilizing the refractive index anisotropy of a ferroelectric liquid crystal molecule in combination with polarizers has been proposed in Japanese Patent Laid-Open No. 56-107216, U.S. Pat. No. 4,367,924, etc. by Clark and Lagerwall. Generally speaking, such a ferroelectric liquid crystal exhibits a chiral smectic C phase (Sm.sup.* C phase) or a chiral smectic H phase (Sm.sup.* H phase), both of which are of a non-spiral structure, in a specific temperature range. In these phases, the liquid crystal is in either a first or second optically stable state with respect to the electric field applied thereto. Further, it has bi-stability, that is, even when the electric field is removed, it maintains the state it has assumed. Furthermore, it is characteristic of a ferroelectric liquid crystal to quickly respond to any change in the electric field, so that it can be expected to be applied to a wide-screened, high-definition display as a high-speed drive, memory-type display medium which can be driven on a simple matrix basis.
Various methods of effecting gradated display by using a ferroelectric liquid crystal have been proposed, including a cell-thickness gradient method, an electric-potential gradient method, and a method using a minute inversion region called domain. It is important for a liquid crystal device using a ferroelectric liquid crystal as mentioned above, having bi-stability, to have an appropriate threshold distribution, from the minimum transmittance (all-black condition) to the maximum transmittance (all-white condition), with respect to a predetermined range of voltage and/or pulse width when a voltage for obtaining a particular half tone is applied. In such a gradation drive, the binary display technique, according to which fluctuations are restrained by utilizing drive margins, is not applicable, and a particular half tone must always be obtainable with respect to a particular write voltage (and pulse width).
A satisfactory gradated display, however, cannot be achieved if there is any hysteresis in the voltage/transmittance characteristic of the liquid crystal cell. In such a case, even when voltages having the same waveform are applied, the resulting transmittance in the case of transition from a dark (black) to a bright (white) state is not the same as that of the reverse.
Techniques related to gradated display are disclosed in U.S. Pat. Nos. 4,712,877, 4,747,671, 4,763,994, and 4,796,980, assigned to the same assignee, to whom the present invention has also been assigned.
Techniques related to liquid crystal orientation and liquid crystal materials are disclosed in U.S. Pat. Nos. 4,639,089, 4,681,404, and 4,721,365, assigned to the same assignee, to whom the present invention has also been assigned.